The best method of calculating the nutritive value of any food consists in estimating how many calories it produces in the body during combustion. One calorie is the amount of heat necessary to raise the temperature of one gramme of water one degree Centigrade. Comparing our system to an oven in which food represents the fuel: Just as oxygen is necessary for combustion in the oven, so we could not carry out the processes of combustion in our system without a plentiful supply of oxygen through our lungs and skin. Also, just as gases produced in a furnace must have free exit, so the carbonic acid, formed in our tissues in the processes of combustion, must be eliminated by the lungs and skin, otherwise auto-intoxication will occur. The above-named authorities have reckoned exactly how many calories the various kinds of food produce in our body. All articles of diet can be classified into three principal groups: proteids, carbohydrates, and fats. According to the above-named authorities one gramme[254] of proteid produces 4.1 calories; one gramme of carbohydrates, 4.1 calories; but one gramme of fat produces more than twice as many, i.e., 9.3 calories. Footnote 254: As well known, one gramme is the equivalent of 15 grains. In order to maintain life without waste or without exposing ourselves to disease, it is necessary to use all three kinds of food; for although many animals, or human beings, can live for a certain time on only one variety of food, most of them would die if this were persisted in for a long time. The total exclusion of proteids especially would produce, after a certain time, a considerable wasting of the body tissues and certain death. According to Voith, it is necessary to take about 100 grammes of albumin a day if we want to avoid waste of body tissue. Proteid food cannot be replaced by either of the other two groups of food. There are a series of facts which show that the estimate given by Voith is perhaps too high. Horace Fletcher has shown by experiments on himself, controlled by Professor Chittenden, that he could live in splendid health with food not containing more than 45 grammes albumin, and of 1600 calories heat value, in twenty-four hours, with a body weight of 186 pounds. As shown by Professor Noorden,[255] in Vienna, a man must take 30 to 34 calories for each kilo (2⅕ pounds) of his bodyweight when he is doing no work, and 34 to 40 calories with light, and 40 to 60 calories with harder work. Accordingly a man weighing 70 kilos would require to take food equivalent to about 2800 calories for light work, and about 3500 to 4000 calories for heavy work. But Fletcher got along well on 1600 calories with a body weight of 186 pounds. However, he lost some weight, 36 pounds, but became healthier and stronger than he was previously. Later on he still further reduced his diet and lived on 38 grammes of albumin and 1581 calories, continuing in perfect health. Footnote 255: